1. Field of the Invention
The present invention relates to a ferromagnetic metal-film-type recording medium and, more particularly, to a method and apparatus for producing a magnetic recording medium of the type mentioned above, wherein durability of a protective layer which is formed after the formation of a magnetic layer so as to improve the performance thereof in practical use, is remarkably improved, as well as to an apparatus suitable for use in carrying out the method.
2. Description of the Related Art
Ferromagnetic metal-film-type recording media have been known which are produced by forming, through vacuum deposition, sputtering, or ion plating or a like method conducted in a vacuum atmosphere, a film of Co, Ni, Fe, or alloys thereof on a polymeric film such as a polymide film or on a substrate which is made of a non-magnetic metal. In these ferromagnetic metal-film-type magnetic recording media, it is possible to remarkably improve recording density as compared with a conventional application type magnetic recording medium. In order to attain a high recording density, it is an important factor to diminish as much as possible any recording/reproduction detects, as well as to minimize spacing losses due to spacing between a magnetic head and a magnetic recording medium. A magnetic recording medium must also have high durability. In order to meet these requirements, hitherto, it has been known to cover a material layer with a protective layer, and, further to form a lubrication layer. A method is also known in which surface contamination is removed by an argon glow discharge process just before a protective layer is formed. FIG. 5 shows a magnetic recording medium in which a protective layer is formed by a conventional plasma CVD (Chemical Vapor Deposition) method, and a lubrication layer is formed by a wet-application method. Numeral 1 denotes a substrate, 2 denotes a ferromagnetic metal-film-type layer formed by a vacuum deposition method, 3 denotes a back coating layer, 4 denotes a protective layer formed by a plasma CVD method, and 5 denotes a lubrication layer formed by a wet-application method. FIG. 6 shows an example of a method of producing a magnetic recording medium so as to form a protective film (a diamond like carbon film) by a plasma CVD method after an argon glow process is performed for cleaning as disclosed in U.S. Pat. No. 4,833,031 . An example of the above-mentioned method and apparatus for producing magnetic recording media will be explained hereinunder with reference to FIGS. 5 and 6. First, a description will be given, with reference to FIG. 6, to an apparatus for forming a protective layer 4 by a plasma CVD method after a conventional argon glow process is performed. Numeral 10a denotes a magnetic recording medium before the formation of a protective layer, which is wound on a supply roller 11. Numerals 12 and 14 denote pass rollers, which rotate in contact with the ferromagnetic thin-film layer 2 on the magnetic recording medium 10. Numeral 13 denotes a main roller which is insulated from the main part of the apparatus and which is capable of feeding the magnetic recording medium 10 in close contact with the ferromagnetic metallic film layer 2 with a predetermined voltage applied therebetween. Numeral 15 denotes a take-up roller, which continuously takes up the magnetic recording medium 10b after formation of a protective layer. Numeral 16 denotes a nozzle for an argon glow-discharge process, 17 denotes an electrode, 18 denotes an argon gas introduction port, 19 denotes a power supply for a glow discharge process. These components 10 to 19 in cooperation constitute an argon glow discharge process unit. Numeral 20 denotes a nozzle for plasma CVD, 21 denotes an electrode, 22 denotes a gas introduction port, 23 denotes a plasma-generating power supply. These components in cooperation constitute a plasma CVD processing unit for forming protective layers. Numeral 24 denotes a bias power supply, which applies a voltage between the main roller 13 and the ferromagnetic thin-film layer 2 of the magnetic recording medium 10. This power supply is disposed outside a vacuum container together with the power supply 19 for glow discharge processing and the plasma-generating power supply 23. Next, a will be described of producing a magnetic recording medium by a conventional CVD method which employs an apparatus constructed as mentioned above. The magnetic recording medium 10a, in the condition before the formation of the protective layer 4 is unwound from the supply roller 11 and is advanced through the bus roller 12. The magnetic recording medium 10a is further continuously fed in close contact with the main roller 13 with a voltage applied between itself and the main roller 13. On the other hand, an argon glow discharge is generated from the nozzle 16 as the result of a supply of argon gas from the argon gas introduction port 18 before the formation of a protective layer. The glow is produced by the electrode 17 for argon glow discharge and the power supply 19. Foreign matter on the surface magnetic layer of the magnetic recording medium 10a is removed. Immediately after this, a plasma ion current is generated as the result of the supply of reactive gas flowing in from the gas introduction port 22 and of a voltage applied from the plasma-generating electrode 21 and the plasma-generating power supply 23, and this current is fed to the plasma nozzle 20. It reaches the ferromagnetic metallic film layer 2 of the magnetic recording medium 10a, and a protective layer 4 is formed. The magnetic recording medium 10b with the protective layer 4 formed thereon is taken up by the take-up roller 15 through the pass roller 14.
However, in the above-mentioned conventional method, not only is the foreign matter on the magnetic layer surface removed in the argon glow discharge process before the formation of the protective layer, but damage is caused to the surface of the magnetic layer by the argon ion current at glow discharge time. This leads to decreased still durability and resistance to corrosion, resulting in unsuitability for practical use. Further, as disclosed in U.S. Pat. No. 5,013,583, there has been provided a method of forming a Ca.sub.3 O.sub.4 protective film by a vapor deposition method or sputtering method. However, such methods cannot provide a continuous protective film having a thickness of 100 Angstroms so as to improve a corrosion resistance thereof. The vapor deposition method cannot provide an adequate adhesion strength. The sputtering method causes the magnetic layer to be damaged by ionizing the sputter gas, and the protective film includes metallic atoms. As a result, still durability of a video tape recorder is deteriorated by solidification of metal (contained in the protective layer of the tape) on the head of the recorder.